The Earth Science Picture of the Day (EPOD) highlights the diverse processes and phenomena which shape our planet and our lives. EPOD will collect and archive photos, imagery, graphics, and artwork with short explanatory captions and links exemplifying features within the Earth system. The community is invited to contribute digital imagery, short captions and relevant links.

Coriolis and Hurricanes

These two satellite photographs show hurricanes in the Northern and Southern Hemispheres. Due to Earth's rotation, winds that travel long distances appear to curve. This is called the Coriolis effect. Some sources identify the "Coriolis force." But this is a fictitious force. Winds, like other currents in fluids tend to travel in a straight line along the pressure gradient. That is, winds blow from locations of high pressure to low pressure. In a sense, winds "want" to blow straight downhill. But we observe the winds from a rotating frame of reference - 360° per day or 15° per hour at the poles. At the Equator, this becomes a linear motion with no apparent deflection. So the rotational component of Earth's motion decreases from the poles to the equator.

In the northern hemisphere, winds appear to curve to the left, that is, counterclockwise. Observations of the Coriolis effect in toilets and sinks cannot be documented simply because the distance the water travels is too small. Even tornadoes are not consistent in their rotational motion. But over long distances, regional weather systems do show a consistent curvature. This also applies to major ocean currents. Once again, it's actually the air and water "trying' to flow straight while Earth rotates beneath them.

As a result of the Coriolis effect, one of the storms above shows the counterclockwise wind direction characteristic of Northern Hemisphere storms. The other image shows a hurricane with Southern Hemisphere clockwise rotation. (Keep in mind that winds blow toward the center of the storm when air pressure is the lowest.) Which is which? Answer -- the one on the left is a Southern Hemisphere system.

I would clarify the writeup for the June 5 EPOD where it states, "In the northern hemisphere, winds appear to curve to the left, that is, counterclockwise." It is true that northern hemisphere cyclones spin counterclockwise, and that in the cyclone the winds are deflected to the left. However, it is not true that the Coriolis effect causes winds in the northern hemisphere to be deflected to the left. In fact, the Coriolis effect causes winds to be deflected to the right, and this ends up causing the counterclockwise (left-deflected) rotation of cyclones.

From the frame of reference of the northern hemisphere, any object moving horizonally is deflected to the right by the Coriolis effect. See the link below: although it calls Coriolis a "Force" (see my note below), it nonetheless is correct in its description of the deflection direction. Wind heading toward a low pressure zone is deflected to the right (from the northern hemisphere frame of reference). As the wind nears the low pressure zone, the low pressure becomes greater than the Coriolis effect, and the wind ends up spinning counterclockwise around the low pressure zone (again, see the link below). As the wind spins around the low pressure zone, its movement is a deflection to the left. This deflection is contrary to the direct Coriolis effect, yet caused indirectly by the right-deflection of the Coriolis effect.

By the way, the Coriolis effect can be considered a "force" if one chooses a location on the moving earth (away from the equator) as one's frame of reference. Similarly, the "force of gravity" is not truly a "force", it is more correctly "acceleration due to gravity". (If I take as the frame of reference the surface of the earth, then I feel an effective force of gravity pulling me down-- but this is an artifact of my frame of reference.) The same can be said about "centrifugal force"-- from a turning frame of reference, centrifugal force is real.

link: http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/crls.rxml

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Sincerely,

Mike
Michael Gutfeldt
mgutfeldt@gmail.com

**** June 6, 2006 **** Author's response:

Thank you, Mike.
Frames of reference isn’t something I wanted to get into on the EPOD. Fortunately, Dr. Einstein had no such compunctions.
“winds appear to curve to the left...” That’s why I put in the word “appear.”
I would have no objection to your text being added to the site in the archive. (...with your name)
...